1. Field of the Invention
The present invention relates to a muffler connected to an exhaust system of an engine for an automobile or the like, and particularly relates to a resonant-type muffler capable of efficiently reducing exhaust noise of a particular frequency.
2. Description of the Related Art
There has heretofore been known a muffler which includes: an exhaust pipe provided in an exhaust system of an engine for an automobile; and an outer cylinder covering at least part of the exhaust pipe (see Japanese Utility Model Application Laid-open No. 6-22521 (Patent Document 1)). In the muffler, a space between the exhaust pipe and the outer cylinder is formed as a resonance chamber, and the inside of the exhaust pipe and the resonance chamber communicate with each other through a hole formed in a pipe wall of the exhaust pipe and a connection passage [a half-arc member and either a narrow pipe or a cylinder body]. The connection passage is formed on an outer peripheral surface of the exhaust pipe in the axial direction thereof and is connected to the hole on a base end side and to the resonance chamber on a tip end side.
The muffler is referred to as a resonant-type muffler. The resonant frequency of the resonance chamber is set at a particular frequency, so that the muffler can efficiently reduce exhaust noise of frequencies the same as and close to the resonant frequency.
Meanwhile, in the muffler described in Patent Document 1, if the frequency of exhaust noise to be reduced is to be shifted toward a lower frequency, the resonant frequency of the resonance chamber is accordingly shifted toward the lower frequency. In this case, the following techniques are conceivable as techniques of shifting the resonant frequency of the resonance chamber toward the lower frequency:                (1) The capacity of the resonance chamber is increased;        (2) The passage cross-sectional area of the connection passage is decreased; and        (3) The passage length of the connection passage is increased.        
However, the technique (1) above increases the volume of the resonance chamber, and thus has a problem of causing interference with vehicle components around the muffler. Technique (2) above suppresses air flow into and out of the resonance chamber, and thus makes resonance less likely to occur in the resonance chamber.
Thus, it is desirable to use the technique (3) above which increases the passage length of the connection passage for shifting the resonant frequency of the resonance chamber toward the lower frequency. However, if the connection passage is extended in the axial direction with no contrivance (e.g., is extended in a right direction in FIGS. 2, 3, 4, and 6 of Patent Document 1) in the muffler, there arises a problem. That is, since a space for extending the connection passage in the axial direction is small, the resonant frequency of the resonance chamber cannot be sufficiently shifted toward the lower frequency, and thus cannot be set at the desired lower frequency.